As stated in our Treasure Trails Guide, your map piece is of a spot on the way to Mort'ton. Starting in Mort Myre Swamp make your way towards the Haunted Mine. Then walk North. You need to find the location that's pictured in your map. When you do, take a closer look at the numbers and letters on your map. The numbers on your map are the number of steps you must take, and the letters are the direction you must take the steps in. Starting at the most southern point, follow the steps and when you get to the spot indicated by the X on your map, DIG.